1. Field of the Invention
This invention relates to an analog to frequency convertor.
2. Description of the Prior Art
Analog to frequency convertors are known in many different forms. Where there are high accuracy requirements, for example in static electricity meters, the charge quantity compensation process and the charge changing process are now preferred to other methods.
In the charge quantity compensation process, known for example from Landis & Gyr News 19 (1972) 1 page 13, a measuring current is integrated in an integrator, and each time a given integrator voltage is reached a constant compensation charge is withdrawn from the integrator. A balance is established between the charge supplied and the charge withdrawn, the number of compensating charges per unit of time being proportional to the measuring current. The charge content of the individual compensation pulses represents a measuring constant and can be kept constant very accurately with simple means. On the other hand, special measures have to be taken to prevent the analog to frequency convertor from running idle.
In the charge changing process, known for example from German Offenlegungsschrift No. 1,946,245, the measuring current is similarly integrated in an integrator; each time a certain upper threshold value and a certain lower threshold value of the integrator voltage is reached, the polarity of the measuring current is reversed, thereby reversing the direction of integration. The number of polarity changes per unit of time is proportional to the measuring current. In the charge changing process the value of the integrator capacitance and the difference between the upper and lower threshold value represent measuring constants which are difficult to keep constant with the requisite long term stability. On the other hand idle running of the analog to frequency convertor is prevented automatically. Where such an analog to frequency convertor is used in a static electricity meter, fault currents independent of the reversal of polarity can be partly compensated by the periodic reversal of polarity, thereby extending the measuring range.